Eighteen kerogen samples isolated from the Noshiro GS-1 cores (589-3, 510m) were examined by pyrolysis gas chromatography. Pristene (prist-1-ene) compound was detected in the pyrolyzates of all the kerogens, and the ratio of the pristene to C17 alkane (Pr=/C17) in the pyrogram decreased with increasing burial depth of the samples. The ratio correlated well with vitrinite reflectance (Ro) data. A difference of the particle size of kerogen (>25μm and <25μm) did not affect the Pr=/C17 ratio. Although the number of the samples in this experiment was small and only one well was investigated, it has been suggested that the Pr=/C17 ratio can be used as a useful maturity indicator of kerogen (Ro<0.6%). It would be especially usefull to estimate the maturation level of kerogens containing no vitrinite maceral.